He left on his journey when he was 10.
He's traveled through every region he's gone through for at least a year, except for the Orange Islands. Kanto + Johto + Hoenn = 13. Maybe. Maybe half a year(?) for Orange Islands - 13 in a half. Now he's in Sinnoh. He's been there for at least a few months, but it might have passed his birthday, which they never celebrate, so most likely 14 by now. Though, in the Manga Red(Usually considered Ash) is 16 when he finished his journey. He may most likely be 15 now, and turn 16 at the end of his Sinnoh adventures, and for all we know so far, he might be done with his journey entirely, when he finishes Sinnoh.